The present invention relates to television cameras. More particularly, the present invention relates to the correction of uniformity variations or shading, particularly of black and white signals, resulting from variations of the spot speed, the geometry and convergence of the three primary color images of a video camera.
A three-tube colour camera subdivides the light received during shooting using an optical system, to form three so-called primary images on the targets of three red, green and blue tubes. These images are then detected. A difficulty encountered in this system is the centering of these images and the superimposing of the three scans, centering and superposing is necessary to cause passage of spots at the same instant onto the three imges of the same point of the object. The fine adjustments of these images areas and the geometry distortions of the scans and the objects may be adequate at times, but the precision required is such that the geometry adjustments are not maintained over a period of time. Adjustment systems exist for restoring the superimposed position (convergence) by modifying the scans or sweeps of the tubes with respect to one another, so that the resulting signals can, by superimposing, restore a good quality image. Generally the geometry correction is calculated for the green channel by comparison with an electronic test pattern, and is applied to the three channels of red, green and blue. Supplementary convergence corrections are then calculated relative to the green channel taken as a reference and are applied to the red and blue channels.
At present, automatic adjustment systems exist which are controlled by microprocessors and modify the line and frame sweep speeds of each of the three tubes as a function of the calculated corrections. In one of these known systems, correction signals are established after a measuring cycle and are superimposed on the horizontal and vertical sawtooth sweep signals of the pickup tubes. These correction signals accelerate or decelerate the speed of the scanning or sweep spot and thus obtain appropriate centering and superimposing of the three colours. The correction signals are typically filtered in an analog or digital manner, or smoothed by calculations. However, these correction signals still induce discontinuities in the sweep sawtooth. These discontinuities lead to spots or stains in the resulting image, which can be intensified by subsequent video processing of the grey expansion type, automatic servocontrol of the black level, intensifying of the contours, etc.
Independently of the uniformity defects induced in the signal from a television camera by spot speed variations, there are other uniformity defects, whereas the scanning of an image of uniform luminance and hue should give primary signals with constant values. In practice, such constant values are not obtained for various reasons linked with faults in the optics, e.g. non-uniform sensitivity of the scanners, to stray light, etc. A correction of such faults or defects, called spots or stains, is conventionally accomplished by adding to the video signals correction signals produced at the line frequency and at the frame frequency for the correction of black spots and by multiplying the video signals by correction signals for the correction of white spots. These correction signals can be regulated manually or automatically on the basis of a test pattern, or by a combination of both of these. This so-called stain or spot correction is carried out directly on the signals from the pickup tubes and consequently make it possible to maintain high (white) and low (black) levels of the video signal at constant values for each of the three tubes.
However, as stated hereinbefore, apart from these spots or stains which are corrected in a conventional manner, the spots or stains linked with speed variations of the sweep spot produced as a result of the automatic correction of the convergence and geometry by an automatic correction system are added thereto. These latter stains are much more clearly defined than the first mentioned stains. This phenomenon is particularly sensitive in the vertical direction of the image. Thus, in the horizontal direction, the sweep correction is carried out with a timing greatly exceeding the horizontal sweep frequency, so that the correction signals are smoothed in horizontal deflection coils. This smoothing is much less pronounced in the vertical direction, because the speed variations in question produce interlining variations of the frame. These defects have always existed, but the use of automatic means and digital geometry corrections have led to spots, whose geometrical shape, linked with the manner of measuring the errors and correcting them, is visually unfavourable. Thus, the correction signals resulting from the scan lead to variations in steps, which can be large. Smoothing has made it possible to reduce these variations, but the increases are visible on the image, particularly in the transitions between the elementary scan patterns in the uniform zones. Thus, if at a given instant the speed of the spot is increased compared with the preceding instant and the quantity of electrons to be discharged on the target is the same (hue and luminance are equal to those of the preceding instant), the resulting scan signal increases, which leads to overbrightness in the restored image. Conversely when the speed of the spot decreases, the scanned video signal decreases.
In order to solve this problem, the apparatus according to the invention detects the variations in the sweep speed of at least one scanning spot, e.g. that of the green channel, because the variations in the two other channels are the same, except for the differential convergence corrections. These variations correspond to the derivative of the sweep signal of the scan spot. This signal is identical in phase and in shape to the defects noted on the tubes. This signal is then added, with an appropriate gain and sign, to the correction signals of the black and white spots of the three channels or directly to the corrected video signals.
The present invention further defines an apparatus which corrects for shading induced in video signals from a television camera when the shading is caused by the variations in speeds of scanning spots. An automatic correction circuit includes a microprocessor and a correction storage device which stores values used for correcting geometry and convergence errors. The values are updated during measurement cycles and are read under normal operations. These increases are also used for correcting for the shading. A digital to analog converter and an integrator amplifier, as well as a combination circuit, correct these values as well as receiving video signals from the pickup tubes.